This invention relates generally to a method for providing a metal contact to a buried layer of a semiconductor device and, more particularly, to the fabrication of an integrated circuit having electrical isolation between functional elements thereof and metal contacts which directly contact a buried layer.
Heretofore, a number of methods have been used to provide contacts to buried layers in semiconductor devices; however, most of these have been proven to be disadvantageous in various respects.
In a first method, one in which a P-type epitaxial layer is grown over an N+ type buried layer, which may serve for example as the collector of a transistor, it is necessary to deep-diffuse a region through the P-type epitaxial layer in order to contact the buried layer. This is commonly referred to as sink-diffusion. However, this is a high temperature step normally taking place at a temperature of approximately 1,000.degree. C for a period of a few hours. It is well known that any high temperature step can be damaging during the fabrication of semiconductor devices since severe stresses are placed on the devices, resulting in a reduced yield. Further, when using a deep-diffusion (sink-diffusion) technique, it is necessary that the buried layer be doped with a dopant which will only minimally be affected by the subsequent high temperature deep diffusion process. An example of a dopant compatible with the deep diffusion process would be antimony.
In a second method, when dealing with an N-type epitaxial layer, deep diffusion is not necessary and contact may be made to the top of the N-type epitaxial layer. However, since the N-type epitaxial layer normally has a low impurity concentration, it is necessary to contact large areas of the buried collector, thereby increasing collector resistance.
Neither of the above described approaches are suitable for making a low-noise device. Thus, all the prior art methods suffer from some deficiency. They are either time-consuming, damaging to the semiconductor material due to excessive exposure to extreme temperatures thereby reducing yield, or require large contact areas which in turn reduces design densification of devices.